1. Field of the Invention
The present invention relates to a discharge bulb suitable for a light source of a vehicle lamp, the discharge bulb including a ceramic arc tube body (an arc tube made of ceramic) inside which discharge electrodes are provided so as to face each other and a light-emitting substance (such as a metal halide) is sealed therein together with a starting rare gas. In particular, the present invention relates to a high-pressure discharge bulb for a vehicle lamp, to which a direct-current lighting method is applied.
2. Description of the Related Art
A discharge bulb having a glass arc tube body (an arc tube made of a glass tube) is generally used as a light source of a vehicle headlamp. However, there has been a problem that a corrosion of the glass tube is caused to progress due to a light-emitting substance (a metal halide) sealed inside the glass tube, thereby causing a blackening or a transparency losing phenomenon to appear, whereby a proper light distribution cannot be obtained and the life of the glass bulb does not last so long.
Therefore, in recent years, discharge bulbs have been proposed which includes a ceramic arc tube body in which discharge electrodes are provided to face each other and a light-emitting substance is sealed therein together with a starting rare gas (see, e.g., JP-A-2004-362978). More specifically, as shown in FIG. 9, molybdenum pipes 212 are joined to each of the thin tube portions 201 of a ceramic tube 200 on respective sides by a metallization, electrode rods 214 are inserted through the interior of each of the molybdenum pipes 212 in such a manner that distal end portions of the electrode rods 214 project into the interior of (a discharge emission chamber S of) the ceramic tube 200, and rear end portions of the electrode rods 214 are joined (welded) to respective rear end portions of the molybdenum pipes 212 projecting from the ceramic tube 200, whereby respective end portions (the thin tube portions 201 communicating with the discharge emission chamber S) of the ceramic tube 200 are sealed (welded portions 218 being electrode sealing portions at respective ends of the arc tube body). Since the ceramic tube 200 is stable with respect to the light-emitting substance (metal halide) that is sealed therein, the ceramic arc tube body has a longer life than the glass arc tube body. Reference numeral 215 denotes a minute gap between the molybdenum pipe 212 and the electrode rod 214.
Currently, in view of downsizing a system and reducing a production cost, there is also a movement to develop a high-pressure discharge bulb using a direct-current lighting method. Such a high-pressure discharge bulb for a liquid crystal back light was found as a high-pressure discharge bulb in which a ceramic arc tube body is lit by a direct-current lighting method (see, e.g., JP-A-10-188893).
However, the high-pressure discharge bulb shown in JP-A-10-188893 is for a liquid crystal back light, and hence, it cannot be used as a light source of a vehicle lamp which desirably requires a white luminescence that is free from color unevenness.
More specifically, in case of the direct-current lighting method, a cataphoresis phenomenon is generated in which a light-emitting substance is separated into positive and negative ions in association with polarities of electrodes while being lit. Color unevenness in an electric arc attributed to the cataphoresis phenomenon is unavoidable. Nevertheless, such a color unevenness can be mitigated (absorbed) to some extent by using a ceramic tube which has a low linear transmission factor (e.g., 80% or less) compared with a glass tube so that an emitted light is diffused. However, in the process of developing a high-pressure discharge bulb using the direct-current lighting method for use in a light source of a vehicle lamp, there was a primary problem that the color unevenness in the electric arc is not sufficiently mitigated (absorbed) only by using such a ceramic tube. Furthermore, irrespective of the fact that a suitable amount of light-emitting substance is sealed inside the discharge emission chamber S, there was a secondary problem that performance characteristic is decreased and a luminous flux is decreased.